Karan's Chapter 7!

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DHCP

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Service that provides a way for client computer to request IP address assignment. Delivers necessary configuration information to clients. Traces its origins back to an earlier protocol named BOOTP. Servers can manage one or more ranges of IP addresses

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WhenTCP/IP is configured on client computer (How DHCP works)

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The Obtain an IP address automatically option is the only necessary set-up element

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Next time workstation attempts to access network (How DHCP works)

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It broadcasts a DHCP address request to the network because it has no IP address

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AllDHCP servers present on same broadcast domain (How DHCP works)

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Receive request and send back message that indicates a willingness to grant an address lease

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Client (How DHCP works)

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Accepts address lease offer and sends packet to server that extended offer

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Inreply (How DHCP works)

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Server proffers an IP address for a specific period of time that the client uses thereafter

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When half the lease period expires (How DHCP works)

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Client attempts to renew the lease

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Leases

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“Loans” of an address for a specific amount of time

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Leases that range from one to three weeks

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Typical on networks in which machines seldom move and the workforce is stable

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Lease periods

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Average between one and three days for networks on which roving workers come and go regularly

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Lease periods of four to eight hours

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Common on ISP networks

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DHCP client (DHCP Software Elements)

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Enabled at client machine when you select the Obtain an IP address automatically option

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DHCP server (DHCP Software Elements)

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Manages address pools and configuration data

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DHCP relay agent

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Intercepts address requests on local cable segment. Repackages requests as a unicast to one or more DHCP servers

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Manual address lease (DHCP Lease Types)

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Administrator assigns an IP address manually

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Dynamic address lease (DHCP Lease Types)

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DHCP server assigns addresses for specific periods of time

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How DHCP integrates with DNS

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Server addresses are advertised using DNS. All address updates must be entered manually. Client addresses usually come into play only when:

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Email addresses of the form [e-mail] must be

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resolved

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Two types of IPv4 address auto configuration mechanisms

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DHCP & Automatic Private IP Addressing (APIPA)

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Automatic Private IP Addressing (APIPA)

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Dynamic configuration of IPv4 link: local addresses. Address block169.254.0.0/16: Reserved for this use. Used by interfaces as a fail over mechanism to self- assign an IPv4 address. APIPA address will not allow routed communications to hosts on other networks

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DHCP Discovery Occurs when

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DHCP client broadcasts a request for an IP address

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DHCP Discovery

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Relies on an initial DHCP broadcast

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DHCP servers

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PING an address before offering it to the client

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Windows clustering

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Allows two or more servers to be managed as a single system

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DHCP Discovery Uses four packets

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DHCP Discover packet, DHCP Offer packet, DHCP Request packet, DHCP Acknowledgment packet

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Discover Packet

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During DHCP Discovery process. Client broadcasts Discover packet that identifies client’s hardware address. Header contains: Source IP address 0.0.0.0

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Preferred address for Discover Packet

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Typically the last address the client used

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Offer Packet

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Send by DHCP server to offer IP address to DHCP client. Includes IP address that is offered to the client Sometimes, answers to the requested options in the DHCP Discover packet

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Request Packet

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Once Offer packet is received. Client can either accept offer by issuing a DHCP Request packet, or Reject offer by sending a DHCP Decline packet. Typically Client sends a Decline only if it receives more than one Offer

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Acknowledgment Packet

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Sent from server to client to indicate the completion of the four-packet DHCP Discovery process

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Acknowledgment Packet Response

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Contains answers to any configuration options requested by client in the previous Request packet

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Address Renewal Process

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When DHCP client receives an address from a DHCP server. Client also receives a lease time and notes the time that the address was received

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Lease time for Address Renewal Process

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Defines how long the client can keep the address. In middle of the lease period client starts a renewal process.

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Renewal Time (T1)

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Defined as the time that the client tries to renew its network address

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The Renewal packet

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Is unicast directly to the DHCP server

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Default value for T1

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0.5 * duration_of_lease (i.e., lease time)

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Rebinding Time (T2)

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Time that client begins to: Broadcast a renewal request for an extended lease time from another DHCP server

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DHCP specification, RFC2131

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defines default value for T2 as 0.875 * duration_of_lease

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DHCP Address Release Process

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Client should release its address by Sending a DHCP Release packet to the server

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DHCP Release packet

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Sent over UDP. DHCP server does not send any acknowledgment

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If client does not send the DHCP Release packet

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DHCP server automatically releases the address at the lease expiration time

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DHCP Packet Structures Fields

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Operation Code (OPCODE) or Message Type Field Hardware Type Field
Hardware Length Field
Hops Field
Transaction ID Number Field
Seconds Since Boot or Seconds Elapsed Field
Flags Field
Client IP Address Field
Your IP Address Field
Server IP Address Field
Gateway or Relay Agent IP Address Field
Client Hardware Address Field
Server Host Name Field
Boot File Field
DHCP Option field

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DHCP Option 53: Message Type

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Only DHCP option required in all DHCP packets.

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DHCP Option 53: Message Type

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Indicates general purpose of any DHCP message

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DHCP boot sequence uses these message types:

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DHCP Message Type 1: Discover (client to server). DHCP Message Type 2: Offer (server to client). DHCP Message Type 3: Request (client to server). DHCP Message Type 5: ACK (server to client)

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DHCP clients Must broadcast

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service requests until they obtain IP addresses

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DHCP clients Use unicast when

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addressing after they obtain an address for a local

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DHCP servers

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Examine DHCP packets coming from clients

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DHCP boot up process

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Relies heavily on broadcasts, but most routers do not forward broadcasts

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Relay agent function

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Typically loaded on a router connected to the segment containing DHCP clients

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Relay agent device

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Configured with the address of the DHCP server. Can communicate using unicast packets directly with that server

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Two basic approaches to IPv6 auto configuration

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Stateless and Stateful

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Stateless auto configuration

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Simply presents required router configuration information to all comers

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Stateful auto configuration

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DHCPv6 server must maintain awareness of the status or state of its pool of available addresses

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Stateless address auto configuration

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ND protocol allows routers to be configured to present the minimum information a host needs when joining a network link

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Stateful address auto configuration

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DHCPv6 is much like DHCPv4 under IPv4

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Both stateless and stateful address rely on

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dedicated servers to hold databases of information about hosts and their IP and other configuration parameters

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Combination of stateful and stateless address auto configuration

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Referenced as DHCPv6 stateless.

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Routers on the local link can be configured to

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provide pointers to DHCPv6 servers. Router provides the network prefix, and DHCPv6 server provides the DNS server information

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Functional States of an IPv6 Auto configured Address

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Tentative addresses, Valid addresses, preferred addresses, deprecated addresses, invalid addresses.

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Tentative addresses

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Occur as a node initializes an interface on an IPv6 network segment or link

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Valid addresses

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Usable based on the Valid Lifetime field in the Prefix Information option of an RA or the Valid Lifetime field in the DHCPv6 IA Address option

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Preferred addresses

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Usable based on the Preferred Lifetime field in the Prefix Information option of an RA or the Preferred Lifetime field in the DHCPv6 IA Address option

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Deprecated addresses

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Allow nodes to continue to function while they renew the lease on their addresses

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Invalid addresses

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Cannot be used as either the source or destination address when the valid lifetime expires

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Node interface identifiers (IDs)

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Used to ensure that the IPv6 address is unique among all other IPv6 addresses
Generally 64 bits long

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Three most common sources of node interface identifiers

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Modified EUI-64 format, Random number generator to create a 64bit number, Cryptographically Generated Addresses (CGA) process

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RFC4941

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Node using SLAAC will compute an additional IPv6 address known as the “temporary” address

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Temporary address

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Assigned “preferred” status. Used for all outbound communications from the node

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DHCPv6

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Defined in RFC 3315. Uses UDP ports 546 and 547

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DHCPv6 uses two specific multicast addresses:

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FF02:1:2 & FF05::1:3

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DHCP Unique Identifier (DUID) methods

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DUID-LLT, DUID-EN, DUID-LL

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Numerous DHCPv6 message types

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Occur between nodes, servers, and relay agents

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DHCPv6 Stateful Message Exchange

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1.Host sends a Router Solicitation
2. Router replies with a Router Advertisement
3. Host sends a Solicit message
4. DHCPv6 server replies with an Advertise message
5. Host sends a Request message
6. DHCPv6 server sends the host a Reply message with an IPv6 address
7. Host sends a Router Solicitation
8. Router replies with a Router Advertisement
9. Host sends an Information-Request message
10. DHCPv6 server sends the host a Reply message with the other available configuration options

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DHCPv6 Relay Message Exchange

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1.Host sends a Router Solicitation
2. Router replies with a Router Advertisement
3. Host sends a Solicit message
4. Router relay-forwards Solicit message to server
5. DHCPv6 server relay-replies router with an Advertise message
6. Router replies with Advertise message to the host
7. Host sends a Request message
8. Router relay-forwards Request message to server
9. DHCPv6 server relay-replies router with a Reply message with IPv6 address and other options
10. Router replies with the Reply message to the host

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In IPv6 Auto configuration process basic steps nodes create

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an interface ID

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In IPv6 Auto configuration process basic steps FE80 is

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prepended to the interface ID

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In IPv6 Auto configuration process basic steps Node sends

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NS message with wits link-local address as the destination

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In IPv6 Auto configuration process basic steps if an NA is not recieved

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then the address is considered unique

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In IPv6 Auto configuration process basic steps host sends an RS to

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the all routers multicast address FF02;;2

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In IPv6 Auto configuration process basic steps if an RA is not received

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the host stars the stateful auto configuration process

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IPv6 Auto configuration Process Basic steps
If an RA is received

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host examines the RA message looking for variables and flags

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IPv6 Auto configuration Process Basic steps
If L flag is set to “on,”

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host adds the network prefix to its prefix cache

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IPv6 Auto configuration Process Basic steps
If a flag is set to “on,”

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then two IPv6 addresses are created

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IPv6 Auto configuration Process Basic steps
Host sends an NS message

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with its global unicast address as the destination

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IPv6 Auto configuration Process Basic steps
If an NA is not received

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the address is considered unique

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IPv6 Auto configuration Process Basic steps

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If M flag is set to “on,” start the stateful auto configuration process

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IPv6 Auto configuration Process Basic steps

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If O flag is set to “on,” start the stateful auto configuration process
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Auto configuration in Microsoft Windows Operating Systems
Basic guidelines:

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Link-local address will be generated using a RNG, If a flag is set to “on” in an RA, the global unicast address will be generated, If a flag is set to “on” in an RA, a temporary global unicast address will be generated, OS sends the RS to find any on-link routers, If no RAs are received, OS starts stateful address process, Manually configuring an IPv6 address does not disable IPv6 address auto configuration

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Microsoft Windows Server 2008 DHCP Scopes
Address scopes

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Define a set of addresses that a DHCP server can assign to clients

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Microsoft Windows Server 2008 DHCP Scopes Super scopes

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A collection of scopes that contain sets of non- consecutive IP addresses

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Source of information needed in setting up a simple DHCP Server

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D-Link Xtreme N Gigabit Router (model number DIR-655)

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One good way to troubleshoot DHCP/DHCPv6

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Use a protocol analyzer, such as Wireshark

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Analyzer when Troubleshooting DHCP

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Can display the sequence of messages that occurs on a network

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Command store lease and/or renew an IP address when Troubleshooting DHCP

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ipconfig /release, ipconfig /renew, ipconfig /release6, ipconfig /renew6

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DHCP

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Provides a way for computers to obtain usable, unique IP addresses and necessary TCP/IP configurations

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From the administrative side DHCP

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makes it easy to define and manage pools of IP addresses

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DHCP’s origins

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Lie in an earlier TCP/IP Application layer protocol, called BOOTP

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DHCP

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Supports manual or dynamic address allocation

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When a DHCP client starts

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It begins the process of DHCP Discovery, during which the client receives an IP address and lease

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DHCP supports a variety of

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message types and options. Only MessageType 53 (DHCP Message) is mandatory for any given DHCP message

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DHCP supports

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Supports manual or dynamic address allocation

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When a DHCP client starts

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It begins the process of DHCP Discovery, during which the client receives an IP address and lease

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DHCP

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Supports a variety of message types and options. Only MessageType 53 (DHCP Message) is mandatory for any given DHCP message

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Because DHCP can ferry a surprisingly large range of configuration information

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The protocol makes use of several message options

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A protocol analyzer is especially effective when

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diagnosing DHCP/DHCPv6 difficulties

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Clients supporting IPv6 have

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new operations for address auto configuration

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DHCPv6 operates much like

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DHCP for IPv4

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DHCPv6 is a completely new service, compared to

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DHCP’s origins